Process for the preparation of an aqueous emulsion of a midblock sulfonated block copolymer

ABSTRACT

The present invention provides an aqueous emulsion and a process for preparing an aqueous emulsion of a midblock sulfonated styrenic block copolymer comprising at least two non-sulfonated polymer end-blocks A and at least one 5 sulfonated block B comprising the steps of providing a cement of a midblock sulfonated styrenic block copolymer in a non-polar solvent wherein the non-polar solvent is a hydrocarbon compound comprising from 5 to 12 carbon atoms with a boiling point of less than 100 degree Celsius or mixture of such compounds, mixing the cement with a co-solvent to form a mixture, emulsifying the mixture optionally in the 10 presence of an emulsifier with water to produce an emulsion, and removing the hydrocarbon solvent and optionally the co-solvent from the emulsion to produce the aqueous emulsion. The resulting emulsion of the midblock sulfonated styrenic block copolymer has relatively small particle diameters.

TECHNICAL FIELD

This invention concerns a process for the preparation of an aqueousemulsion of a midblock sulfonated block copolymer. More in particular,it concerns emulsions of midblock sulfonated block copolymers withrelatively small particle diameters. It also concerns the aqueousemulsion so prepared.

BACKGROUND ART

Midblock sulfonated block copolymers are known. Typically, they aresulfonated polymers based on styrene and/or t-butyl styrene with theformer predominantly used in a midblock, that is subsequently sulfonatedand the latter in the endblocks, that resist sulfonation. These polymersare in a solid state in the presence of water and have both high watertransport properties and sufficient wet strength. These polymers areknown to have excellent barrier properties.

From WO2007010039 a midblock sulfonated styrenic block copolymer isknown. This block copolymer is based on a block copolymer that comprisesat least two polymer end blocks A and at least one polymer interiorblock B wherein each A block is a polymer block resistant to sulfonationand each B block is a polymer block susceptible to sulfonation, andwherein said A and B blocks do not contain any significant levels ofolefinic unsaturation.

Such polymers are now commercially available for instance under thetrademark Nexar® from Kraton Polymers. The typical structure of a Nexarmolecule is a pentablock consisting of two poly(t-butylstyrene) (tBS)blocks, two poly(ethylene/propylene) (EP) blocks (hydrogenatedpolyisoprene), and in the middle a partly sulfonated polystyrene (sPS)block.

Such midblock sulfonated block copolymers are typically delivered tocustomers as a solution of about 10% in a combination of heptane andcyclohexane. For some customers this poses a problem because they arenot used to handling this type of solvent and do not have adequateventilation and disposal systems in place. Organic solvents may causevarious handling problems due to the high volatility and low flame pointof such solvents. The possibility to deliver such polymers as an aqueousemulsion would be a solution. Moreover, a waterborne system is moreenvironmental friendly. Preparing a suitable aqueous emulsion, however,is not without its own problems.

EP2242137 and EP1852928 concern a membrane-electrode assembly forpolymer electrolyte fuel cells. They employ a block copolymer comprisinga polymer block (A) having ion-conductive groups and a polymer block (B)having no ion-conductive groups, both polymer blocks phase-separatedfrom each other, polymer block (a) forms a continuous phase. Inparagraph [0047] of EP2242137 it describes methods of emulsifying theblock copolymer. This is described and illustrated for end-blocksulfonated block copolymers only. End-block sulfonated block copolymersbehave differently from the midblock sulfonated block copolymers. Amethod for preparing an aqueous dispersion of a midblock sulfonatedblock copolymer is therefore not disclosed in EP2242137 or EPI 852928.

The solution inversion emulsification method, when applied on a midblocksulfonated styrenic block copolymer dissolved in a hydrocarbon solventlike cyclohexane/heptane, produces a rather coarse aqueous emulsion withrelatively big average particle size of about 7.0 μm or larger (asdetermined by laser diffraction spectroscopy). Ideally, the averageparticle size should be about 2.0 μm or smaller. Smaller particles havebetter film forming properties.

Using a series of homogenizers is not attractive. The homogenizers are asubstantial capital investment. Moreover, the properties of the polymerare affected by the physical homogenization. Furthermore, there is aserious risk of loss of material when mechanically reducing the size ofthe particles from coarse (7.0 μm or greater) to fine (2.0 μm orsmaller). Finally, the pH of the so produced emulsions is very low (<2).A corrosive emulsion may adversely affect the equipment used.

Ideally it should be possible with ordinary equipment to produceemulsions with reduced average particle size (well) below 7.0 μm. Thisshould be possible, even when using a hydrocarbon solvent wherein thepreceding polymerization has been performed. Expressed differently, whenusing a typical solution of a midblock sulfonated styrenic blockcopolymer dissolved in a hydrocarbon solvent such as cyclohexane,heptane or a mixture thereof, it would be ideal if the average particlesize could be reduced by a factor of 4 or greater without having toinvest heavily in equipment or having to replace the solvent.

SUMMARY OF INVENTION

The present invention provides a process for preparing an aqueousemulsion of a midblock sulfonated styrenic block copolymer comprising atleast two non-sulfonated polymer end-blocks A and at least onesulfonated block B, comprising the following steps:

-   -   a) providing a cement of said midblock sulfonated styrenic block        copolymer in a non-polar solvent, wherein the non-polar solvent        is a hydrocarbon compound comprising with a boiling point of 49        to 99° C. or mixture of such compounds;    -   b) mixing the cement of step a) with a co-solvent to form a        mixture;    -   c) emulsifying the mixture of step b), optionally in the        presence of an emulsifier, with water to produce an emulsion;    -   d) removing the hydrocarbon solvent and optionally the        co-solvent from the emulsion to produce the aqueous emulsion,        wherein the non-polar solvent has a Hansen polarity parameter        (δp) smaller than 2.0 (expressed in √MPa), wherein the        co-solvent is a polar aprotic solvent or polar protic solvent        with a Hansen polarity parameter (δp) in the range of 2.8 to 15,        preferably in the range of 5.0 to 12, and a Hansen hydrogen        bonding parameter (δh) in the range of 4.0 to 27 (expressed in        √MPa). Preferably, the co-solvent has a boiling point of at most        99° C., whereby an aqueous emulsion may be provided with little        or no (organic) solvents.

The co-solvent may be removed, albeit that trace amounts (up to 1500ppm) will remain. Co-solvent may also be left in the emulsion bypurpose, to allow the co-solvent to improve the film forming propertiesof the emulsion. The emulsion made by the process of the invention istherefore different from an emulsion made without a co-solvent.Accordingly, the present invention provides an aqueous emulsion of amidblock sulfonated styrenic block copolymer comprising at least twonon-sulfonated polymer end-blocks A and at least one sulfonated block B,having a solids content in the range of from 10-30, suitably from 10-15%by mass calculated on the mass of the e, wherein the average particlesize of the particles of the midblock sulfonated styrenic blockcopolymer is at most 2.0 μm, containing trace amounts of a co-solvent,wherein the co-solvent is a polar aprotic solvent or polar proticsolvent with a Hansen polarity parameter (δp) in the range of 2.8 to 15,preferably 5.0 to 12, and a Hansen hydrogen bonding parameter (δh) inthe range of 4.0 to 27 (calculated in √MPa).

Thus the inventors found that by making use of specific solvents duringemulsification, the average particle diameter of aqueous midblocksulfonated styrenic block copolymer emulsions could be decreased. Thefinal diameter depends on the type and amount of solvent added.

DESCRIPTION OF EMBODIMENTS

Accordingly, the present invention broadly comprises emulsions ofmidblock sulfonated styrenic block copolymers that are solids in water.The block copolymers comprise at least two non-sulfonated polymerend-blocks A, and at least one interior styrenic polymer block Bcarrying sulfonyl groups and/or derivatives thereof. Optionally themidblock sulfonated styrenic block copolymer may comprise one or moreinterior polymer blocks D that have a glass transition temperature ofless than 20° C. Such polymers are known from WO2007010039, fromEP2242137 and others.

The expression “resistant to sulfonation” is sometimes used with respectto the end blocks A. This means that less than about 10 mol % of all theavailable sulfonyl groups in the sulfonated styrenic block copolymer arein the A blocks. The expression “resistant to sulfonation” if used withrespect to the blocks D, will mean that less than about 15 mol % of allthe available sulfonyl groups in the sulfonated styrenic block copolymerare in the D blocks.

The expression block copolymer refers to a polymer havingdistinguishable blocks. These blocks have different properties.Typically, the block copolymer has multiple, distinct transitiontemperatures. An important difference between end-block sulfonated blockcopolymers and midblock sulfonated styrenic block copolymers is that inthe latter the A blocks can provide a hydrophobic matrix, even if themidblock sulfonated block copolymer is in contact with water. Thepolymer behaves as if it is cross-linked. This is important for thestability of e.g. membranes made from such sulfonated block copolymers.The interior B block on the other hand will be hydrophilic as a resultof the sulfonyl groups or the derivatives thereof that are present inthis block. The D blocks, if any, may have properties ranging fromhydrophobic to hydrophilic, provided, they do not adversely affect thestability of the articles made of the sulfonated block copolymers whenin contact with water. Preferably, they are hydrophobic.

The midblock sulfonated block copolymers may be linear or branched.Preferred structures have the general configuration A-B-A, (A-B)n(A),(A-B-A)n, (A-B-A)nX, (A-B)nX, A-B-D-B-A, A-D-B-D-A, (A-D-B)n(A),(A-B-D)n(A), (A-B-D)nX, (A-D-B)nX or mixtures thereof, where n is aninteger from 2 to about 30, X is coupling agent residue and A, B and Dare as defined hereinbefore.

A distinguishing feature of block copolymers which have been selectivelysulfonated in an interior block is that they can be formed into objectshaving a useful balance of properties that have heretofore beenunachievable, including strength even when equilibrated with water,water vapour transport behaviour, dimensional stability, andprocessability. The hydrophobic blocks and their position at the ends ofthe block copolymer chain contribute to the wet strength, dimensionalstability and processability of these polymers and objects formed fromthem. The sulfonated block(s) positioned in the interior of thecopolymer allow effective water vapour transport. The combinedproperties afford a unique material ideally suitable for coatings.

The sulfonated styrenic block copolymers may be made from correspondingunsulfonated styrenic block copolymers by sulfonation (reaction with SO₃or with a C₂ to C₈ acyl sulfate, as described in WO200710039,incorporated herein by reference). These unsulfonated styrenic blockcopolymers may be defined by the same structural formulae, wherein A andD have the same meaning (since A is resistant to sulfonation and Dpreferably is resistant to sulfonation), but wherein B′, instead of B,is the corresponding block before sulfonation.

Most preferred structures are either the linear A-B-A, (A-B)₂X,(A-B-D)₂X and (A-D-B)₂X structures or the radial structures (A-B)_(n)Xand (A-D-B)_(n)X where n is 3 to 6. The block copolymers prior tosulfonation are typically made via anionic polymerization, cationicpolymerization or Ziegler-Natta polymerization.

Preferably, these unsulfonated block copolymers are made via anionicpolymerization. It is recognized that in any polymerization, the polymermixture will include a certain amount of A-B diblock copolymer, inaddition to any linear and/or radial polymers. The most preferredstructure is A-D-B-D-A, made by sulfonating the correspondingA-D-B′-D-A.

Preferably

-   -   a. each A block independently is a polymer block having an        apparent number average molar mass between 1,000 and 60,000,        more preferably between 5,000 and 40,000, still more preferably        between 7,000 and 20,000; and/or    -   b. each D block, if present, independently is a polymer block        having an apparent number average molar mass between 1,000 and        60,000, more preferably between 2,000 and 40,000, still more        preferably between 5,000 and 20,000, and/or    -   c. each B block independently is a polymer block having an        apparent number average molar mass between 10,000 and 300,000,        more preferably between 15,000 and 200,000, still more        preferably between 19,000 and 100,000.

As used in this specification and claims, the term “molar mass” refersto polystyrene equivalent, or apparent, molar mass of the polymer orblock of the copolymer, measured with gel permeation chromatography(GPC) using polystyrene calibration standards, such as is done accordingto ASTM D5296-11. GPC is a well-known method wherein polymers areseparated according to molecular size, the largest molecule elutingfirst. The chromatograph is calibrated using commercially availablepolystyrene molar mass standards. The detector used is preferably acombination ultraviolet and refractive index detector. The molar massexpressed herein is measured at the peak of the GPC trace.

Preferably, the sulfonated styrenic block copolymer has a B blockcontent in the range of 10 to 85 percent by mass (% m), preferably inthe range of 20 to 60% m, more preferably 25 to 50% m, calculated on themass of the sulfonated block copolymer. Such block copolymers will beinsoluble in water and non-dispersible in water. The hydrophobic unitsof the end blocks and optional D blocks contribute to the blockcopolymer's insolubility. Furthermore, if the B block mass contentapproaches high values where the sulfonated block copolymers becomesoluble, hydrophobicity of the entire styrenic block copolymer can beadjusted by incorporating hydrophobic monomer units into the interiorblocks, including A blocks as well as B blocks.

An important feature of the sulfonated styrenic block copolymers used inthe emulsions of the current invention is that they have sufficientsulfonic groups per molecule (which definition includes salts and acidderivatives that allow transport of water). Preferably the sulfonatedblock copolymer has a content of sulfonic groups in the range of 0.2 to4.0, preferably 0.3 to 3.0, more preferably 0.5 to 2.5 mmole per grampolymer. This is also referred to as Ion Exchange capacity in sulfonicgroups by mass. (The value in meq/g and mmol/g coincide, since thecharge of the sulfonyl group is one).

With respect to the sulfonated styrenic block copolymers that are madefrom an unsulfonated styrenic block copolymer, preferably each A blockindependently is made of monomers that resist sulfonation. Such monomersmay be selected from: (i) para-substituted styrenes; (ii) ethylene;(iii) alpha olefins of 3 to 18 carbon atoms; (iv) 1,3-cyclodienes; (v)conjugated dienes; (vi) acrylic esters, (vii) methacrylic esters, and(viii) mixtures of monomers (i) to (vii). Where sulfonation conditionsare selected that will react with any residual olefinic unsaturation(e.g., in case of block copolymers based on diene monomers in theendblocks), the olefinic unsaturation is preferably removed, forinstance by hydrogenation. To ensure the blocks A provide a strongmatrix, preferably the blocks have a glass transition temperature inexcess of 30° C. For instance, if diene monomers are used, thenpreferably they are polymerized in a 1,4-fashion. More preferably, eachA block comprises a polymer or a copolymer of a para-substitutedstyrene.

The para-substituted styrenes that are considered suitable monomers maybe selected from para-methylstyrene, para-ethylstyrene,para-n-propylstyrene, para-iso-propylstyrene, para-n-butylstyrene,para-sec-butylstyrene, para-iso-butylstyrene, para-t-butylstyrene,isomers of para-decylstyrene, isomers of para-dodecylstyrene andmixtures of the above monomers. Preferred para-substituted styrenes arepara-t-butylstyrene and para-methylstyrene, with para-t-butylstyrene(tBS) being most preferred. Monomers may be mixtures of monomers,depending on the particular source. It is desired that the overallpurity of the para-substituted styrenes be at least 90% m, preferably atleast 95% m, and even more preferably at least 98% m of the desiredpara-substituted styrene used as monomer.

Other preferred monomers that may form the basis of an A block includeethylene; propylene, butylene, hexane or octane; 1,3-cyclohexadiene,1,3-cycloheptadiene and 1,3-cyclooctadiene; 1,3-butadiene and/orisoprene (preferably hydrogenated); and various (meth)acrylic esters.

If the B block is made from sulfonated monomers, then the A blocks mayalso comprise other monomers that would normally undergo sulfonation.Preferably the sulfonated polymer is made by sulfonating an unsulfonatedblock copolymer, and therefore preferably the A blocks contain little orno monomers that would normally undergo sulfonation.

Thus, the A blocks may contain up to 15 mole percent of the vinylaromatic monomers mentioned for incorporation in the B blocks. In someembodiments, the A blocks may contain up to 10 mole percent, preferablythey will contain only up to 5 mole percent, and particularly preferablyonly up to 2 mole percent of the vinyl aromatic monomers mentioned forincorporation in the B blocks.

However, in the most preferred embodiments, the A blocks will contain novinyl aromatic monomers mentioned for incorporation in the B blocks.Accordingly, the sulfonation level in the A blocks will be 0 or close to0 (expressed in mole percent of the total monomers in the A block). Inthe preferred embodiments therefore the A blocks will provide a stronghydrophobic matrix even if the sulfonated styrenic block copolymer is incontact with water.

The sulfonated styrenic block copolymer may optionally comprise one ormore D blocks, providing elasticity. Preferably, said D blocks comprisea polymer or copolymer or a hydrogenated polymer or copolymer of aconjugated diene or a mixture of the conjugated diene with acopolymerizable monomer. The conjugated diene is preferably selectedfrom isoprene, 1,3-butadiene and mixtures thereof, of which between 20and 80 mole percent is built into the (co)polymer in a 1,2-additionfashion. Most preferably, said D blocks are hydrogenated blocks ofpolymerized isoprene (EP). Another example of a suitable D block wouldbe an acrylate or silicone polymer. In still another example, the Dblock would be a polymer of isobutylene.

The advantage of a D block or D blocks is an increasedelasticity/toughness of the products made from the sulfonated blockcopolymer. Whereas the sulfonation level in the A blocks is preferably 0or close to 0 mole percent, some sulfonation of the D block or D blocksis permissible. The level of sulfonation depends on a number of aspects,amongst others relating to the size of the D block(s) and the size ofthe A blocks.

Furthermore, the sulfonated styrenic block copolymers comprise at leastone B block, wherein each B block is made of sulfonated monomers ormonomers that can be sulfonated after polymerisation. Sulfonatedmonomers, preferably sulfonated styrenic monomers, include the varioussulfonyl-vinylstyrene monomers.

Preferably the sulfonated styrenic block copolymers are made bysulfonating the corresponding unsulfonated styrenic block copolymerwherein the or each B′ block is made of monomers that can be sulfonatedafter polymerisation. These monomers are preferably vinyl aromaticmonomers selected from (i) unsubstituted styrene, (ii) ortho-substitutedstyrenes, (iii) meta-substituted styrenes, (iv) alpha-methylstyrene, (v)1,1-diphenylethylene, (vi) 1,2-diphenylethylene and (vii) mixturesthereof, with styrene being most preferred. During sulfonation all orpart of the vinyl aromatic monomers are sulfonated, resulting in—forinstance—sulfonated polystyrene (sPS).

Each B block or B′ block may be a homopolymer or copolymer. Forinstance, this may be a random or tapered copolymer of a sulfonated orunsulfonated vinyl aromatic monomer with other vinyl aromatic monomersand/or with one or more conjugated dienes. These blocks may also have acontrolled distribution of monomers, similar to the polymers disclosedin U.S. Published Patent Application No. 2003/0176582, which disclosureis herein incorporated by reference. Use of a copolymer may beadvantageous to influence the amount of sulfonic groups in the B blocks.This is particularly advantageous when the sulfonated block copolymer ismade by sulfonating selectively the interior block of an unsulfonatedblock copolymer.

For instance, in the interior block of the unsulfonated styrenic blockcopolymer used for the preparation of the sulfonated styrenic blockcopolymer the mole percent of vinyl aromatic monomers which areunsubstituted styrene, ortho-substituted styrene, meta-substitutedstyrene, alpha-methylstyrene, 1,1-diphenylethylene, and/or1,2-diphenylethylene in the or each of the interior B′ blocks is fromabout 10 to about 100 mole percent, preferably from about 25 to about100 mole percent, more preferably from about 50 to about 100 molepercent, even more preferably from about 75 to about 100 mole percentand most preferably 100 mole percent. Note that the ranges can includeall combinations of mole percents listed herewith.

As for the level of sulfonation, typical levels are where each B blockcontains one or more sulfonic functional groups. Preferred levels ofsulfonation are from10 to 100 mole percent based on the mole percent ofvinyl aromatic monomers which are unsubstituted styrene,ortho-substituted styrene, meta-substituted styrene,alpha-methylstyrene, 1,1-diphenylethylene, and 1,2-diphenylethylene ineach B block, more preferably from 20 to 95 mole percent and even morepreferably from 30 to 90 mole percent. The level of sulfonation can bedetermined by titration of a dry polymer sample, which has beenredissolved in tetrahydrofuran with a standardized solution of NaOH in amixed alcohol and water solvent. When the level of sulfonation is belowthe mentioned limit, then the conductivity is adversely affected.Sulfonation close to 100% may be too much effort to be economicallyfeasible. The preferred ranges provide the more attractive balance inproperties and economic feasibility.

With regard to the anionic polymerization process to prepare thestyrenic block copolymers, frequently used solvents wherein the blockcopolymers are dissolved or emulsified to form a cement, are hydrocarboncompounds with a boiling point of 49 to 99° C., with a Hansen polarityparameter (δp) smaller than 2.0 (expressed in √MPa). These non-polarsolvents, more than polar solvents allow for the preparation of styrenicblock copolymers. Typical examples include cyclic alkanes, such ascyclopentane, cyclohexane, cycloheptane, and cyclooctane. Consequently,the midblock sulfonated block copolymers are frequently supplied as asolution using said solvents optionally in admixture with otherhydrocarbon solvents such as heptane as fluid. For some customers thisposes a problem because they are not used or equipped to handle thistype of solvent. The possibility to deliver the midblock sulfonatedblock copolymer as an emulsion would be a solution. Apart from this, theprocess by which the emulsion is generated would give the producer ofthe midblock sulfonated block copolymers the possibility to recover theorganic solvents for potential re-use.

The current invention concerns the preparation of an aqueous emulsion,i.e., a mixture of the solid phase and the aqueous phase that arenormally immiscible. In this case it concerns a stable emulsion, whereinthe solid phase does not separate quickly over time, with fine polymerparticles.

Emulsification procedures are well-known and common procedures may beapplied in the current invention. Quite some literature has beenpublished about emulsification by high shear. A lot of publicationsconcentrate on separate mechanisms playing a role, e.g. droplet breakup,surface tension, dynamic surface tension. During the process ofemulsification, when droplets become smaller, it will becomeprogressively more difficult to decrease the size of the dropletsfurther. Droplet size may also depend on the type of surfactant used, ifany. Viscosity can also be of importance. The midblock sulfonated blockcopolymers of the current invention typically have a relatively highviscosity, making it more difficult to from emulsions with smallparticle sizes.

The midblock sulfonated block copolymers can be emulsified via indirector by direct emulsification. With the former procedure water is added tothe dissolved polymer, first creating a water in oil emulsion, whichafter addition of more and more water inverts into an oil in wateremulsion.

Direct emulsification may be carried out by adding the dissolved polymerto water while stirring. Initially large particles are formed, which maythen be broken down for instance by the high shear. It may be usefulthat a surfactant is present in the water phase, to lower theinterfacial tension between the oil and the water phase, and to providestabilization of the particles formed. The particle diameters may to acertain extent be influenced by adjusting the shear rate and by changingthe surfactant concentration. However, the desire to reduce the averageparticle diameter remains.

In order to create e.g. a homogeneous film, it is important to createemulsions with small average particle diameters. The smaller the better.The average particle diameter is suitably smaller than about 2.0micrometer, more suitably smaller than about 1.0 micrometer.

The preferred procedure for preparing emulsions of sulfonated blockcopolymers is by direct emulsification. Preferably, a solution of thesulfonated block copolymer in a volatile organic solvent (with a boilingtemperature below that of water) and water are mixed and turned into anemulsion, typically with the aid of a homogenizer. For instance, for labscale experiments an Ultra-turrax™ T25 or T50 may be used; forlarge-scale experiments a Danfoss™ VLT5000 rotor-stator setup may beused. Interestingly, as described hereafter, the sulfonated blockcopolymers do not require the presence of a surfactant to create astable emulsion.

Having mixed the solution of the sulfonated block copolymer and water,next the organic solvent is stripped. This may be at atmosphericpressure, whilst heating above the boiling temperature of the organicsolvent used for the preparation of the solution of the sulfonated blockcopolymer, or at a reduced pressure. For example, for lab scaleexperiments the solvent may be stripped from an emulsion first atatmospheric pressure at the temperature close to boiling water, followedby a further reduction of the residual solvent in a rotary evaporatorsetup.

Solids content is the amount of sulfonated block copolymer in theaqueous emulsion, calculated by weight. Preferably, the solids contentis as high as feasible, to keep transportation costs low. On the otherhand, the emulsion has to remain stable and sufficient fluid to bemanageable by the users. Preferably, the resulting emulsion has a solidscontent in the range of 5 to 70% wt., preferably in the range of 10 to50% wt.

Moreover, the resulting emulsion has preferably a content of hydrocarbonsolvents of less than 5% wt., preferably less than 1% wt.

For the purpose of the present invention, it has been found that theaverage particle diameter may be reduced in size by a factor of about4.0 or greater by the presence of a particular co-solvent. This is notmerely a dilution effect (which might provide a reduction of size by afactor of less than 2.0).

Surprisingly, it has been found that certain co-solvents have anadvantageous effect on the average particle diameter. These co-solventsmay be selected on the basis of the Hansen Solubility Parameters.

The Hansen solubility parameter values are based on dispersion bonds(δd), polar bonds (δp) and hydrogen bonds (δh). These containinformation about the inter-molecular interactions with other solventsand also with polymers, pigments, nanoparticles, etc. Because numericalvalues are used, comparisons can be made rationally by comparingnumbers. For example, acetonitrile is much more polar than acetone butexhibits slightly less hydrogen bonding.

Typically, the Hansen parameter is expressed in either √MPa or in√cal/ml). As indicated, the co-solvent is a polar aprotic solvent orpolar protic solvent with a Hansen polarity parameter (δp) in the rangeof 2.8 to 15, preferably 5.0 to 12, and a Hansen hydrogen bondingparameter (δh) in the range of 4.0 to 27 (expressed in √MPa).

An extensive list of solvents, albeit expressed in √(cal/ml), isdisclosed on http://www.stenutz.eu/chem/solv24.php?sort=1. A selectionthereof has been copied here below, arranged by δp and converted to √MPa(multiplied by the factor √4.2, i.e., 2.05):

Name CAS nr δp δh cyclohexane 110-82-7 0.0 0.2 heptane 142-82-5 0.0 0.0toluene 108-88-3 1.4 2.0 decanol 112-30-1 2.7 10.0 diethyl ether 60-29-7 2.9 5.1 2-methylpropyl acetate 110-19-0 3.7 6.3 butyl acetate123-86-4 3.7 6.3 2-ethoxyethyl acetate 111-15-9 4.7 10.6 2-butoxyethanol111-76-2 5.1 12.3 ethyl acetate 141-78-6 5.3 7.2 2-butanol  78-92-2 5.714.5 2-methyl-1-propanol  78-83-1 5.7 16.0 butanol  71-36-3 5.7 15.8tetrahydrofuran 109-99-9 5.7 8.0 2-propanol  67-63-0 6.1 16.4 1-propanol 71-23-8 6.8 17.4 methyl acetate  79-20-9 7.2 7.6 3-pentanone  96-22-07.6 4.7 acetic acid  64-19-7 8.0 15.5 ethanol  64-17-5 8.8 19.42-butanone (“MEK”)  78-93-3 9.0 6.0 2-ethoxyethanol 110-80-5 9.2 14.31,2-propanediol  57-55-6 9.4 23.3 2-propanone (“aceton”)  67-41-1 10.47.0 ethanediol 107-21-1 11.0 26.1 N,N-dimethylacetamide 127-19-5 11.510.2 methanol  67-56-1 12.3 20.7 N-methylpyrrolidon 872-50-4 12.3 7.2N,N-dimethylformamide  68-12-2 13.7 11.3 diethylene glycol 111-46-6 14.720.5 water 7732-18-5  16.0 42.3

Suitable co-solvents are therefore those having a δp similar or greaterthan diethyl ether, preferably similar or greater than 2-butoxyethanoland similar or smaller than diethylene glycol, preferably similar orsmaller than methanol. Note moreover that the co-solvent shouldpreferably also have a boiling point below that of water.

The most important feature of the co-solvent is the polarity component,δp. Preferably, this is in the range of from 5.1 (2-butoxyethanol) to11.5 (N,N-dimethylacetamide). This includes, for instance,tetrahydrofurane (5.7); 1-propanol (6.8); 2-butanone (9.0) and2-propanone (10.4). These co-solvents provide surprisingly good results,with best results being achieved with 1-propanol.

The effective amount of co-solvent depends on the solids content and onthe selection of hydrocarbon solvent used in the cement. Thus, in casethe content of midblock sulfonated styrenic block copolymer is low,e.g., 5% m on the cement, then only a minor amount of co-solvent isneeded, e.g., 3% m on the mixture of cement and co-solvent. In case thesolids content is high, then a greater amount of co-solvent is needed;up to 60% m of the mixture of cement and co-solvent. For instance, in a12% m solution of a midblock sulfonated styrenic block copolymersufficient 2-butanone may be added that the final mixture has a 8% msolids content, with a mass ratio between hydrocarbon solvent andco-solvent of 2:3. Preferably, the mass ratio between the hydrocarbonsolvent and the co-solvent is in the range of 10:1 to 1:2, preferably5:1 to 2:3, more preferably 3:1 to 1:1.

In addition to the co-solvent and the optional detergent, it may beuseful to include special solvents (non-volatile organic compounds asdefined in 1999/13/EC and 2004/42/CE in Europe, and/or non-hazardous airpollutants as defined in the US Clean Air Act) as coalescing agents thatlower the Minimum Film Forming Temperature such as N-methylpyrrolidon,tripropylene glycol n-butyl ether (Arcosolv™ TPNB) various glycolethers, glycol ether esters, or ester alcohols (e.g., Eastman™ EEH) andmany other commercially available solvents, which thereby help toachieve a uniform film, coating or other article.

Together or separate from the above coalescing agents, various otheradditives may be added to the emulsion in their common amounts. Suchadditives include pigments, antioxidants, stabilisers, antifreeze,biocides, catalysts, etc.

The current invention also concerns a film cast from the emulsiondefined above, as well as a method for casting the film. The method forcasting the film comprises casting and drying said emulsion, wherein acoalescing agent is used when drying the film of the emulsion and/orwherein the temperature is increased to above the minimum film formingtemperature. The current invention also concerns a membrane made from afilm cast from the emulsion defined above, as well as a coating madefrom the inventive emulsion.

INDUSTRIAL APPLICATION

The emulsions may be used, for instance, for the preparation of moisturepermeable membranes and coatings. These may find use in for instancefuel cells, all separation equipment wherein membranes are used and inclothing, energy recovery ventilation, water purification.

The emulsions may also be used for other articles.

EXAMPLES ILLUSTRATING THE INVENTION

Emulsions were Prepared Using the Following Materials:

MD9200 A sulfonated styrenic block copolymer pentablock copolymer with astructure tBS-EP-sPS-EP-tBS, wherein the tBS blocks each have an averageMW of about 10,000, the EP blocks each have an average MW of about12.000 and the interior sPS block has an average MW of about 22.000,with an ion exchange capacity (IEC) of 2.0 mmol/g. (10.5% m solids in anapolar solvent mixture of 1:1 cyclohexane/heptane) MD9150 A sulfonatedstyrenic block copolymer pentablock copolymer with a structuretBS-EP-sPS-EP-tBS as set out in FIG. 1, wherein the tBS blocks each havean average MW of about 10,000, the EP blocks each have an average MW ofabout 12.000 and the interior sPS block has an average MW of about22.000, with an ion exchange capacity (IEC) of 1.5 mmol/g. (13.0% msolids in an apolar solvent mixture of 1:1 cyclohexane/heptane) X100C₁₄H₂₂O(C₂H₄O)_(9.5), a nonionic surfactant sold under the trademarkTriton ™ which has a hydrophilic polyethylene oxide group (on average ithas 9.5 ethylene oxide units) and a hydrocarbon lipophilic orhydrophobic group. SDS Sodium dodecylsulfate (C₁₂H₂₅SO₄Na), an anionicsurfactant MEK Methylethylketone or 2-butanone (technical grade, min99%) THF Tetrahydrofuran (technical grade, min 99%) acetone 1-propanone(technical grade, min 99%) Xylene Dimethylbenzene (isomeric mixture, min99%) Toluene Methylbenzene (technical grade, min 99%) 1-propanolTechnical grade, min 99%)

Equipment used in the experiments was an UItraturrax™ T25. A CombimagRCT water bath from IKA was used, as well as a rotovap RV05, also fromIKA.

The solids content of the emulsions was determined by drying a sample inan oven (Gallenkamp Plus oven) at 130° C. for 15 minutes. The averageparticle size diameter was determined using a Beckman Coulter LS230laser diffraction particle size analyzer. Residual solvent content ofthe emulsion was measured by GC head space, using 2,2-dimethylbutane asinternal standard (3.170 gram in 1 litre THF). The Trace GC Ultra headspace gas chromatograph was from Interscience.

Example 1

20 grams of MD9200 solution were mixed 30 grams of water in the T25 at13500 rpm. After stripping off the solvent under reduced pressure atelevated temperature a stable emulsion was obtained with a solidscontent of 7% m and having an average particle diameter of 2.64micrometer.

Example 2a, b, Comparative

Experiment 1 was repeated. 20 grams of MD9200 solution were mixed with7.5 grams of an apolar solvent (cyclohexane, toluene), and 30 grams ofwater in the T25 at 13500 rpm. The results after stripping off thesolvent and the majority of the co-solvent under reduced pressure atelevated temperature are shown in the below Table. It can be seen that astable emulsion may be obtained by dilution (experiment 2a) or use of anapolar solvent. However, the results in terms of particle size are notnearly good enough.

Example 3

Experiment 1 was repeated. 20 grams of MD9200 solution were mixed with7.5 grams of co-solvent (Methyl Ethyl Ketone), and 30 grams of water inthe T25 at 13500 rpm. After stripping off the solvent and the majorityof the co-solvent under reduced pressure at elevated temperature astable emulsion was obtained with a solids content of 7% m and having anaverage particle diameter of 0.76. micrometer.

Examples 4-6, in Accordance with the Invention

Example 3 was repeated with different co-solvents. The results are shownin the table below. A significant average particle size reduction isachieved with a co-solvent meeting the Hansen requirements as defined inthis specification. In particular 1-propanol provides excellent results.

Example 7a, b, in Accordance with the Invention

Experiment 3 was repeated. 20 grams of MD9200 solution were mixed with7.5 grams of MEK as well as with a surfactant 0.2 grams of X100, or 0.2grams of SDS respectively), and 30 grams of water in the T25 at 13500rpm. The results after stripping off the solvent and the majority of theco-solvent under reduced pressure at elevated temperature are shown inthe below Table. Excellent results were achieved, indicating that theco-solvent and surfactant may also be used in combination.

Examples 8-9

Experiments 1 and 3 were repeated, but now with MD9500. The improvementby addition of a proper solvent is mirrored.

Co-solvent Av. Particle Example Polymer (amount in % m) diameter (μm) 1MD9200 None 2.64 2a MD9200 cyclohexane (27%) 2.31 2b MD9200 toluene(27%) 2.42 3 MD9200 MEK (27%) 0.76 4 MD9200 THF (27%) 0.79 5 MD9200acetone (27%) 1.0 6 MD9200 1-propanol (27%) 0.44 7a MD9200 MEK (27%),X100 0.61 7b MD9200 MEK (27%), SDS 0.43 8 MD9150 None 6.7 9 MD9150 MEK(27%) 0.60

The results above illustrate that the presence of a co-solvent has asignificant effect on the average particle diameter.

The most preferred co-solvent is 1-propanol.

1. A process for preparing an aqueous emulsion of a midblock sulfonatedstyrenic block copolymer comprising at least two non-sulfonated polymerend-blocks A and at least one sulfonated block B, comprising thefollowing steps: a) providing a cement of said midblock sulfonatedstyrenic block copolymer in a non-polar solvent, wherein the non-polarsolvent is a hydrocarbon compound comprising from 5 to 12 carbon atomswith a boiling point of less than 100° C. or mixture of such compounds;b) mixing the cement of step a) with a co-solvent to form a mixture; c)emulsifying the mixture of step b), optionally in the presence of anemulsifier, with water to produce an emulsion; d) removing thehydrocarbon solvent and optionally the co-solvent from the emulsion toproduce the aqueous emulsion, wherein the non-polar solvent has a Hansenpolarity parameter (p) smaller than 2.0 (expressed in √MPa), and theco-solvent is a polar aprotic solvent or polar protic solvent with aHansen polarity parameter (δp) in the range of 2.8 to 15, and a Hansenhydrogen bonding parameter (δh) in the range of 4.0 to 27 (expressed in√MPa).
 2. The process of claim 1, wherein the co-solvent has a Hansenpolarity parameter (δP) in the range of 5.0 to 12 (expressed in √MPa).3. The process of claim 1, wherein the co-solvent is selected fromacetone, tetrahydrofuran, methyl ethyl keton, 1-propanol.
 4. The processof claim 1, wherein the co-solvent has a boiling point of less than 100°C.
 5. The process of claim 1, wherein the midblock sulfonated blockcopolymers are either linear or branched, and preferably have thegeneral configuration A-B-A, (A-B)n(A), (A-B-A)n, (A-B-A)nX, (A-B)nX,A-B-D-B-A, A-D-B-D-A, (A-D-B)n(A), (A-B-D)n(A), (A-B-D)nX, (A-D-B)nX ormixtures thereof, where n is an integer from 2 to about 30, X iscoupling agent residue, wherein A represents the non-sulfonated styrenicpolymer end-block, B represents the interior styrenic polymer blockcarrying sulfonyl groups and/or derivatives thereof, and D represents aninterior polymer block D that has a glass transition temperature of lessthan 20° C.
 6. The process of claim 5, wherein the midblock sulfonatedblock copolymer is linear and has the structures A-B-A, (A-B)2X,(A-B-D)2X and (A-D-B)2X structures or is radial and has the structures(A-B)nX and (A-D-B)nX where n is 3 to
 6. 7. The process of claim 5,wherein a. each A block independently is a polymer block having anapparent number average molar mass between 1,000 and 60,000, morepreferably between 5,000 and 40,000, still more preferably between 7,000and 20,000; and/or b. each D block, if present, independently is apolymer block having an apparent number average molar mass between 1,000and 60,000, more preferably between 2,000 and 40,000, still morepreferably between 5,000 and 20,000, and/or c. each B blockindependently is a polymer block having an apparent number average molarmass between 10,000 and 300,000, more preferably between 15,000 and200,000, still more preferably between 19,000 and 100,000.
 8. Theprocess of claim 1, wherein the the mass ratio between the hydrocarbonsolvent and the co-solvent is in the range of 10:1 to 1:2, preferably5:1 to 2:3, more preferably 3:1 to 1:1.
 9. The process of claim 1,wherein the cement has a solids content between 5 to 60, preferably 10to 30% by mass.
 10. The process of claim 1, wherein in step (a) ahomogenizer, preferably a high shear homogenizer is used.
 11. An aqueousemulsion of a midblock sulfonated styrenic block copolymer comprising atleast two non-sulfonated polymer end-blocks A and at least onesulfonated block B, having a solids content in the range of from 10-30,suitably from 10-15% by mass calculated on the mass of the emulsion,wherein the average particle size of the particles of the midblocksulfonated styrenic block copolymer is at most 2.0 μm, containing up to1500 ppm calculated on the mass of the emulsion of a co-solvent, whereinthe co-solvent is a polar aprotic solvent or polar protic solvent with aHansen polarity parameter (δp) in the range of 2.8-15, preferably5.0-12, and a Hansen hydrogen bonding parameter (δh) in the range of 4.0to 27 (calculated in √MPa).